As demand for higher quality pipes or tubes grows in recent years, there is an increasing trend that nondestructive test standards for the pipes or tubes (hereinafter referred to as “pipes” when deemed appropriate) are becoming more stringent.
For example, a seamless pipe, which is a typical pipe, is manufactured by punching a billet with a piercer to form a hollow shell and rolling the hollow shell with a mandrel mill or the like. The seamless pipe has flaws having various tilt angles (hereinafter referred to as “tilted flaws” when deemed appropriate) with respect to the axial direction.
A tilted flaw is believed to be caused by deformation in the axial direction of a longitudinal crack originally existing on the billet in the above manufacturing process or transfer of a flaw existing on a guide face of a guide shoe for maintaining a path center of a hollow shell. Therefore, the tilt angle of the tilted flaw with respect to the axial direction of the seamless pipe changes depending on a difference in a pipe diameter of the seamless pipe or a cause for occurrence thereof. That is, there are tilted flaws with various tilt angles on the seamless pipe.
Since there is a trend of tighter service conditions of the seamless pipes from year to year, higher quality is demanded and accurate detection of the above tilted flaws is also sternly demanded.
Conventionally, various methods for detecting the tilted flaws existing on the seamless pipes have been proposed.
In Japanese Laid-Open Patent Publication No. 55-116251 (hereinafter referred to as “Patent Literature 1”), for example, a method for detecting a tilted flaw by arranging an ultrasonic probe at an appropriate position and tilt angle depending on the position and tilt angle of the tilted flaw to be detected is proposed.
However, the method described in Patent Literature 1 has a problem that extremely much time and manpower are needed because the tilt angle of the ultrasonic probe must be changed each time in accordance with the tilt angle of the tilted flaw to be detected. Also, to detect tilted flaws with various tilt angles existing on the seamless pipe in one round of flaw-detecting work, as described above, many ultrasonic probes must be provided, each of which is arranged with a different tilt angle. That is, there are problems that large equipment is required and soaring costs are entailed, in addition to complicated arrangements/settings and calibration of ultrasonic probes.
To solve the problems of the method described in the above Patent Literature 1, a flaw detecting method that applies an ultrasonic phased array probe in which a plurality of transducers (elements for transmitting/receiving ultrasonic waves) are arranged in a single row is proposed in Japanese Laid-Open Patent Publication No. 61-223553 (hereinafter referred to as “Patent Literature 2”). More specifically, ultrasonic shear waves are propagated within the pipe by aligning an arrangement direction of the transducers with the axial direction of the pipe and arranging the ultrasonic probe decentralized from an axial center of the pipe. Then, according to this method, the tilted flaws with the various tilt angles are detected by changing the tilt angle (tilt angle with respect to the axial direction of the pipe) of ultrasonic waves transmitted and received by the ultrasonic probe using electronic scanning that electrically controls transmission/reception time-shift of the ultrasonic wave by each transducer.
However, two main problems (first problem and second problem) shown below exist in the method described in Patent Literature 2.
<First Problem>
FIG. 1 shows a diagram illustrating an example of a relation between the tilt angle (angle formed by an extension direction of the tilted flaw and the axial direction of the pipe) of the tilted flaws and echo intensity according to an ultrasonic testing method applying an ultrasonic phased array probe, verified by an experiment conducted by the inventors of the present invention. More concretely, FIG. 1 shows echo intensity (relative intensity when the echo intensity of a tilted flaw with the tilt angle 0° is defined to be 0 dB) of each tilted flaw when, in a state where an ultrasonic phased array probe equivalent to that described in Patent Literature 2 is arranged with a constant eccentricity from the axial center of the pipe, the tilt angle of the ultrasonic wave is changed by electronic scanning in accordance with the tilt angle of each tilted flaw so that the extension direction of the tilted flaw and a propagation direction (propagation direction viewed from a normal direction of a tangential plane of the pipe including an incident point of the ultrasonic wave) of the ultrasonic wave transmitted by the ultrasonic probe are orthogonal to each other. The inventors of the present invention have found a problem that, as shown in FIG. 1, echo intensity is different depending on the tilt angle of the tilted flaw even if the tilted flaw is of the same size (0.5 mm in depth and 25 mm in length).
As described above, the inventors of the present invention have found that the method described in Patent Literature 2 has the problem that the echo intensity is different depending on the tilt angle of the tilted flaw and there is a possibility that this problem may prevent detection of a harmful flaw or lead to over-detection of minute flaws that need not be detected.
<Second Problem>
If electronic scanning for electrically controlling transmission/reception time-shift of the ultrasonic wave by each transducer of an ultrasonic phased array probe described in Patent Literature 2 is used to change the tilt angle of the ultrasonic wave transmitted and received by the ultrasonic probe, electronic scanning must be repeated as many times as required depending on the tilt angle of the tilted flaw to be detected in a specific area of the pipe. That is, for example, to detect three tilted flaws with different tilt angles, electronic scanning must be repeated three times in the specific area of the pipe, and ultrasonic testing efficiency is reduced to ⅓ when compared with detection of flaws with a unidirectional tilt angle.
More concretely, one round of ultrasonic testing in the specific area of the pipe, though dependent on, in addition to an outer diameter and thickness of the pipe, distance between the ultrasonic probe and the pipe and the like, takes approximately 50 to 100 μsec. That is, the maximum number of times of ultrasonic testing per unit time (ultrasonic testing speed) in the specific area of the pipe is 10,000 to 20,000 times per second. Therefore, a change speed (change frequency) of the tilt angle of the ultrasonic wave by the electronic scanning is also compelled to be about 10,000 to 20,000 times per second or less, and even if the electronic scanning itself is much faster than mechanical scanning, ultrasonic testing efficiency will decrease as the number of tilt angles of the tilted flaws to be detected increases.
As described above, the method described in Patent Literature 2 has the problem that the ultrasonic testing efficiency goes down as the number of the tilt angles of the tilted flaws to be detected increases.
In Japanese Laid-Open Patent Publication No. 59-163563 (hereinafter referred to as “Patent Literature 3”), on the other hand, a method for causing the ultrasonic wave to enter in any direction using a group of transducers arranged in a matrix state in order to detect the tilted flaws with the various tilt angles is proposed. More concretely, an incident direction of the ultrasonic wave is arbitrarily changed by selecting an appropriate number of arbitrary transducers from the group of transducers and by performing electronic scanning for electrically controlling transmission/reception time-shift (driving time) thereof. Then, it is disclosed that patterns to change the incident directions of the ultrasonic wave are stored in advance as a program.
However, the first problem that echo intensity changes in accordance with the tilt angle of each tilted flaw, as described above, is not mentioned and further, in order to solve the problem, nothing is disclosed about which change pattern should be used to change the incident directions of the ultrasonic wave in Patent Literature 3. In addition, there is a problem similar to the second problem of the method described in Patent Literature 2. That is, there is the problem that the ultrasonic testing efficiency decreases because electronic scanning must be repeated as many times as the number of tilt angles of the tilted flaws to be detected.